Neuroanatomy of the mesothoracic ganglion of the cockroach periplaneta americana (L.) II. Median neuron cell body groups

The groups of neuron cell bodies in the midline of the mesothoracic ganglion of adult male Periplaneta americana (L.) were examined using Bodian silver-stained paraffin sections and toluidine blue-stained wholemounts to determine their general architecture, and axonal filling with Procion yellow or cobalt to study pathways of their neuron processes. Filling tracheae with trypan blue provided additional information on tracheation of the ganglion. Seven groups of somata were named according to position: the anterior m edian (AM), mid-dorsal median (MDM ), posterior dorsal median (PDM ), posterior median (PM ), anterior ventral median (AVM ), mid-ventral median (MVM ) and posterior ventral median (PVM ) groups. Each group is characterized by general appearance and position, numbers and sizes of constituent somata, and numbers and pathways of fibre bundles. Groups consist largely of interneurons, with a few efferent, presumed motor, neuron somata present in some. Different functional types of neurons (that is, motoneurons and interganglionic and intraganglionic interneurons) do not appear to be strictly segregated into different groups. The AM, PM , AVM and M VM groups contain apparently only interneurons, either interganglionic (AM), intraganglionic (PM, AVM ), or both together (M V M ). In the MDM group one motoneuron to each side supplies the dorsal root of peripheral nerve 2, four or five PDM cells (equivalent to dorsal unpaired median (DUM ) cells of other species) bifurcate to nerve 5 of both sides of the ganglion and some also branch to nerves 3, 4 and 6, and in the PVM group one cell to each side is the widespread common inhibitory motoneuron to nerves 3-6. Three other large, bifurcating neurons of the PDM group may be efferent but follow a somewhat different pathway and could not be traced into peripheral nerves. Numbers of somata in the seven groups total 200 or more, and intraganglionic interneurons seem greatly to outnumber interganglionic ones. The MDM , AVM , MVM and PVM groups show evidence of an apparently paired origin.

Neural control of circulation in Aplysia. III. Neurotransmitters

In the abdominal ganglion of Aplysia californica, seven motoneurons have been described which modulate the myogenic heart beat and vasomotor tone (28). These neurons mediate their motor effects by chemical transmission. In this paper we have attempted to specify the transmitters of six of these motoneurons. We have 1) studied the effects of several common transmitters on the innervated structures and compared these effects with the effects of firing the motoneurons, 2) examined whether blocking agents influence similarly the effect of a putative transmitter applied to the innervated structure and the effect of firing a motoneuron, and 3) tested the capability of the motoneurons to synthesize the putative transmitters from precursors. The positive inotropic and chronotropic effects of firing the excitor motoneuron RB(HE) were mimicked by perfusion of the heart with serotonin at a low concentration. Cinanserin blocked both the effects of motoneuron excitation and serotonin perfusion. RB(HE) was also shown to synthesize [3H]serotonin from L-[3H]tryptophan injected directly into the cell body. The effects of firing the two LD(HI) heart-inhibitory motoneurons were mimicked by perfusion of the heart with acetylcholine. Benzoquinonium blocked the effects of the inhibitory motoneuron and acetylcholine perfusion. Perfusion with arecoline also inhibited the heart beat. Acetylcholine applied to the arteries mimicked the vasoconstriction caused by the LB(VC) motoneurons. Aortic constriction in response to activity in LB(VC) cells or to acetylcholine was blocked by hexamethonium and curare. The heart inhibitor and vasoconstrictor motoneurons synthesized [3H] acetylcholine from [3H] choline injected into their cell bodies. Thus, as in vertebrates, acetylcholine mediates inhibition to the heart. Unlike vertebrates, however, serotonin mediates excitation to the heart and acetylcholine mediates peripheral vasoconstriction.